1. Field of the Invention
The present invention relates to a transistor including an insulated gate (MOS structure) (for example, an insulated gate bipolar transistor, a power MOSFET or the like). Additionally, an insulated gate bipolar transistor will hereinafter be also referred to as an “IGBT”, and may be also referred to as a “reverse conducting IGBT”. The present invention relates more particularly to a structure of an insulated gate transistor which has a trench gate structure and incorporates a diode functioning as a freewheeling diode (which will hereinafter be also abbreviated as a “FWD”), and a technique for manufacturing the same. The present invention provides for improvement of recovery characteristics exhibited by the diode incorporated in an insulated gate transistor during an operation mode of the diode.
2. Description of the Background Art
In power electronics for driving a motor or the like, under a condition that a rated voltage is 300V or higher, an IGBT is usually used as a switching device because of its characteristics. In using an IGBT as a switching device, a freewheeling diode (FWD) which is connected in parallel with the switching device is also used.
Now, a structure of a typical trench IGBT will be briefly described. In a typical trench IGBT, an N+-type buffer layer is formed on a P+-type collector layer, and an N−-type layer is formed on the N+-type buffer layer. Also, a P-type base region is selectively formed on a surface of the N−-type layer as a result of diffusion of P-type impurities. An emitter region is formed on a surface of the P-type base region as a result of selective diffusion of a high concentration of N-type impurities. Further, a trench passing through the emitter region into the N−-type layer is formed. An oxide film is formed on an inner wall of the trench, and a gate electrode of polysilicon is filled into the trench having the inner wall on which the oxide film has been formed. A portion of the P-type base region which is located between the emitter region and a portion of the N−-type layer just below the emitter region is a channel region. Moreover, an emitter electrode is formed so as to extend over a portion of a surface of the emitter region and a central portion of the surface of the P-type base region, and a drain electrode is formed on a back surface of an N+-type substrate.
Next, operations of the typical trench IGBT having the foregoing structure will be described. Given with the foregoing structure, upon application of a predetermined collector voltage VCE between the emitter electrode and the collector electrode and also a predetermined gate voltage VGE between the emitter electrode and the gate electrode (to turn on the gate), the channel region is inverted to be of an N type, so that a channel is formed. Then, electrons are injected through the channel from the emitter electrode into the N−-type layer. Because of the injection of the electrons, a forward bias is applied between the P+-type collector layer and the N−-type layer (N+-type buffer layer). This is followed by injection of holes from the P+-type collector layer into the N−-type layer, which results in considerable reduction of a resistance of the N−-type layer in the IGBT, to increase a current capacity of the IGBT. In this manner, the injection of holes from the P+-type collector layer serves to reduce the resistance of the N−-type layer in the IGBT. Turning now to transition from an on state to an off state of the IGBT, first, the gate voltage VGE applied between the emitter electrode and the gate electrode during an on state is reduced to 0V or a reverse bias is applied between the emitter electrode and the gate electrode (to turn off the gate anyway) in the foregoing structure. Then, the channel region is returned from the inverted state, i.e., an N−-type state, to a P-type state, and the injection of electrons from the emitter electrode is terminated. Because of the termination of the injection of electrons from the emitter electrode, also the injection of holes from the P+-type collector layer is terminated. Thereafter, the electrons and the holes accumulated in the N−-type layer (N+-type buffer) go out of the N−-type layer toward the collector electrode and the emitter electrode, respectively. Otherwise, the electrons and the holes are recombined to each other, to disappear.
Then, a basic structure of a FWD parallel-connected to the IGBT having the foregoing structure will be described. The diode is formed by forming a P-type anode region on a surface of an N−-type substrate composed of an N−-type layer, and further forming an anode electrode on a surface of the P-type region. Moreover, an N+-type cathode layer, and subsequently a cathode electrode, are formed on a back surface of the N−-type substrate.
Operations of the diode having the foregoing structure will be described. Given with the foregoing structure, after a predetermined anode voltage VAK (forward bias) is applied between the anode electrode and the N−-type layer and the anode voltage exceeds a certain threshold voltage, a forward bias is applied between the P-type anode region and the N−-type layer, to cause conduction in the diode. Then, upon application of a reverse bias between the anode electrode and the N−-type layer, a depletion layer extends from the P-type anode region toward the N−-type layer, so that a reverse breakdown voltage can be retained.
FIG. 38 shows a current waveform exhibited by the diode having the foregoing structure during reverse recovery thereof in transition from an on state to an off state. As known, a reverse current instantaneously flows during transition of a diode from an on state to an off state. A peak value of the reverse current is called a “recovery current Irr”. Diodes of a type that exhibits a relatively slight tilt in change in current from the recovery current Irr to a value of “0” are referred to as “soft recovery” diodes. Also, a power supply voltage is applied to the diode during reverse recovery and a product of the power supply voltage and the current is a “recovery loss”, though showing therefor is omitted in FIG. 38.
In general, a (soft recovery) diode in which both a steady state loss (Vf) in an on state and a loss in reverse recovery (recovery loss) are low and current recovery takes place gently during reverse recovery is required as a rectifier diode.
A typical inverter circuit functions to change a dc voltage into an ac voltage, and includes IGBTs as switching devices and freewheeling diodes (FWDs). The IGBTs and FWDs form four or six elements, to be used for control of a motor. The inverter circuit includes a dc terminal connected to a dc power supply, and causes each of the IGBTs to perform a switching operation to thereby change a dc voltage into an ac voltage, which is then supplied to the motor as a load.
The typical inverter circuit requires such a freewheeling diode as described above because a motor serving as a load is inductive. The inductive load stores energy in a magnetic field generated by a current. Accordingly, change in a current means change in stored energy. In the following description, an energy storage ability of an inductive load will be represented by “L”. Upon interruption of a current flowing through the load, energy stored in L of the load is released by a matter which is attempting to interrupt the current, so that the energy will function to prevent change in the current. Instant release of the energy stored in the L of the motor leads to generation of an electric power which is high enough to degrade performance of an IGBT. Thus, to suddenly interrupt the current which is caused to flow through the motor by the IGBT would make the IGBT inoperable due to the released energy. In view of this, the freewheeling diode is provided, to cause the current flowing through the motor during an off state of the IGBT to freewheel through a bypass path, in order to prevent the current flowing through the motor from being changed under influence of a switching operation of the IGBT. More specifically, a dc power supply and the motor are connected to each other. In this manner, when the IGBT is turned off to stop applying a voltage to the motor, the current flowing through the motor reverses its course to flow through the freewheeling diode as a direct current because of the energy stored in the L of the motor. As a result, the motor is placed in a state equivalent to a state where a reverse dc voltage is applied to the motor. Changing a ratio between a turn-on time period and a turn-off time period of the IGBT leads to change in a ratio between a time period during which a dc voltage is applied and a time period during which a reverse current is flowing. Accordingly, a voltage applied to the motor can be controlled to be uniform. As such, by changing the ratio so as to become sinusoidal, it is possible to allow the IGBT to perform a switching operation to thereby supply an ac voltage from the dc power supply to the motor while preventing the current flowing through the motor from being suddenly interrupted under influence of the switching operation of the IGBT. Because of the foregoing operating manner of the inverter circuit, there is a need of providing the freewheeling diode inverse-series connected to the IGBT, or providing the freewheeling diode anti-parallel connected to the IGBT which is paired with another IGBT. In this regard, a power MOSFET which also has conventionally been used as a switching device does not require additionally providing a freewheeling diode external to the power MOSFET by virtue of circuitry thereof, i.e., because the power MOSFET inherently includes a built-in anti-parallel connected diode. However, a density of a conductible current of the power MOSFET is relatively low, and thus the power MOSFET is unsuitable for high current applications. On the other hand, the IGBT has a structure formed by changing a bottom region of an N+-type layer to an P+-type layer in a substrate of a vertical power MOSFET, and thus a diode is formed between a P+-type collector layer and an N+-type buffer layer in a back surface. A breakdown voltage of the diode in the IGBT is in a range approximately from 20V to 50V. Such voltage is too high for a breakdown voltage of a built-in freewheeling diode. Because of this high breakdown voltage, a barrier which is unsuitable as a freewheeling diode is formed, and thus performance of the IGBT is significantly degraded due to heat generated by the high breakdown voltage applied during freewheeling. For this reason, while an IGBT is advantageous to a power MOSFET in that flow of a high current in a device is permitted, an IGBT still has a disadvantage of requiring a distinct freewheeling diode connected to the IGBT when the IGBT is employed as a switching device of an inverter circuit, in view of its circuitry.
In light of later development of IGBTs than vertical MOSFETs and presence of both advantages and disadvantages described above of each of IGBTs and MOSFETs, to incorporate a diode properly functioning as a freewheeling diode into an IGBT in the same manner as a freewheeling diode is incorporated in a vertical power MOSFET has been recognized as an immediate task in IGBT technologies. To this end, various approaches as disclosed in Japanese Patent Application Laid-Open (hereinafter abbreviated as “JP”) Nos. 2002-314082, 2000-307116, 9-82954, 8-116056, 7-153942, 6-53511 and 6-196705 have ever been proposed.
Out of the above-cited references, JP Nos. 7-153942 and 6-53511 teach a structure in which a freewheeling diode is incorporated in an IGBT. According to those references, a source of electrons is prepared in a back surface and a P-type base layer in a top surface functions as an anode of a diode. In the structure taught by those two references, however, a surface concentration of the P-type base layer of the IGBT must be set to approximately 1E18 because a threshold voltage Vth of the IGBT is determined by the P-type base layer of the IGBT.
On the other hand, in recently developed diodes, anodes tend to have relatively low impurity concentration, approximately 1E17 for example, in order to improve recovery characteristics thereof.
In this regard, the present inventor investigated influence of a surface concentration of an anode on recovery characteristics by simulation using a structure illustrated in FIG. 39. The structure of a diode model used for the simulation includes: an N−-type substrate having a thickness of 170 μm and a resistance of 55 Ω-cm; an N+-type layer which is formed on a back surface of the N−-type substrate and has a thickness of 1 μm and a surface concentration of 6E18; and a P-type anode layer which is formed on a top surface of the N−-type substrate and has a thickness of 3 μm. For the simulation, two situations where the surface concentration of the P-type anode layer is set to 1E17 and 1E18, respectively, were provided. Also, a life time was set to 10 μ sec. A forward voltage (Vf) of the diode under the foregoing condition was 1.23V in the situation where the surface concentration of the P-type anode layer was set to 1E17, while the Vf was 1.07V in the other situation where the surface concentration of the P-type anode layer was set to 1E18. That is, there was an approximately 15% difference. FIG. 40 shows results of the simulation regarding recovery characteristics. From the results of the simulation in FIG. 40, it can be appreciated that there was an approximately 40% difference in recovery current Irr between the situations where the surface concentration of the P-type anode layer was set to 1E17 and 1E18, respectively, and further appreciated that there was a 50% or more difference in Qrr (a sum of reverse current) between the two situations. As is made clear from the results of the simulation in FIG. 40, the surface concentration of the P-type anode layer greatly influences the recovery characteristics of the diode.
JP No. 6-196705 teaches a structure which provides for improvement of recovery characteristics of a built-in diode incorporated in an IGBT, taking into account the above-noted manner. More specifically, JP No. 6-196705 teaches a structure in which a P−-type layer is formed in a P-type layer in a surface, in order to improve recovery characteristics of the built-in diode. In JP No. 6-196705, it is described that a channel width of the IGBT is 17 μm, a channel width of the diode is 5 μm, a surface concentration of a base layer is 5×1E18, and a thickness of the base layer is 5 μm. A width of the base layer is supposed to be 20% of an overall size based on figures of JP No. 6-196705, though the width of the base layer is not explicitly disclosed in JP No. 6-196705. Thus, it is considered that formation of the P−-type layer in the P-type layer in the surface could not bring about significant effects in the structure of JP No. 6-196705. This is particularly true in high current applications. In high current applications, as injection of holes from a highly doped base layer is dominant during recovery of the diode, the structure taught in JP No. 6-196705 could not be so effective in improving recovery characteristics. It is noted that to simply increase the width of the base layer would cause degradation of characteristics about a reverse leakage current and a reverse breakdown voltage. As a conclusion, the teachings of JP No. 6-196705 cannot be deemed to be effective in improving recovery characteristics of a built-in FWD.
Additionally, while the need of improvement of recovery characteristics of a built-in diode as described above arises quite pressingly in an IGBT incorporating a FWD, the same need also arises in a vertical MOSFET (power MOSFET) incorporating a FWD. To improve recovery characteristics of a built-in diode is a common technical task to be accomplished, for an IGBT incorporating a FWD and a vertical MOSFET (power MOSFET) incorporating a FWD.